A mysterious voice telling divers to get out of the water at a marine life
centre has been identified as that of a 15-year-old white whale.

Researchers at the National Marine Mammal Foundation in California had often heard the sound of people talking near the whale and dolphin enclosure, but had never managed to catch the culprits or get close enough to tell what they were saying.

It was only when a diver surfaced from the pen and asked colleagues why they had told him to exit the tank – when in fact no such command had been issued – that the penny dropped.

Rather than belonging to a hidden miscreant or a ghostly supervisor, the strange noises had in fact been emanating from a teenage white whale called NOC.

After recording the sounds, scientists at the research charity discovered that the whale had changed the way it produced noises in order to make its voice sound more similar to those of its keepers.

Despite the fact whales' vocal mechanisms are very different to ours, and come from the nasal tract rather than the larynx, NOC had developed a similar rhythm and had lowered his voice several octaves to bring it closer to the human range.

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The researchers insisted the behaviour was a clear attempt to mimic humans, and suggested the whale may even have been trying to make contact with them.

They said: "Our observations suggest that the whale had to modify its vocal mechanics in order to make the speech-like sounds. Such obvious effort suggests motivation for contact."

There had previously been anecdotal reports of whales sounding like humans, but the findings published in the Current Biology journal are the first to record and analyse the noises.

NOC began speaking in the 1980s after spending seven years in close contact with researchers at the facility. His "speech" reduced after four years and stopped altogether once he reached adulthood. He died five years ago.

Dr Patrick Miller, lecturer at St Andrews University's school of biology, explained that vocal mimicry is more common in younger animals because the mechanisms they use to make sounds are more flexible.

He said: "It is sort of like babbling speech in humans – when children first learn to speak they are making a whole range of different sounds driven by their desire to communicate with people around them.

"It is probably something of a stretch for the animal to make these sounds but given the context of their captivity and close contact with humans, presumably they were very motivated to do it."